There is known, as a color laser printer of an electrophotographic type, a tandem-type color laser printer provided with a plurality of photosensitive drums arranged in parallel in correspondence with toners of four colors of yellow, magenta, cyan, and black.
As such a tandem-type color laser printer, there is proposed a configuration in which a plurality of photosensitive drums are arranged in parallel above a belt unit having an endless intermediate transfer belt and are brought into contact with the intermediate transfer belt from above.
In the proposed configuration, the belt unit further includes a plurality of primary transfer rollers, a secondary transfer roller, and a cleaning unit.
The primary transfer rollers are arranged opposite to the corresponding photosensitive drums across the intermediate transfer belt. The secondary transfer roller is arranged on the downstream side relative to the downstream most primary transfer roller in the traveling direction of the intermediate transfer belt. The cleaning unit is arranged between the secondary transfer roller and the upstream most primary transfer roller in the traveling direction of the intermediate transfer belt.
When the intermediate transfer belt starts to travel in an image-forming operation, developer images of respective colors carried on the surfaces of the photosensitive drums are sequentially primary-transferred from the photosensitive drums onto the intermediate transfer belt. As a result, a color developer image is formed on the intermediate transfer belt. Thereafter, when the color developer image on the traveling intermediate transfer belt has reached a position opposite to the secondary transfer roller, a paper sheet is supplied between the secondary transfer roller and the intermediate transfer belt. As a result, the color developer image is secondary-transferred onto the paper sheet. The paper sheet onto which the color developer image has been formed is conveyed to a fixing unit, followed by fixing of the color developer image onto the paper sheet.
After completion of the secondary transfer of the color developer image onto the paper sheet, transfer residual toner remains on the peripheral surface of the intermediate transfer belt. When the transfer residual toner remaining on the surface of the traveling intermediate transfer belt has reached to a position opposite to the cleaning unit, the transfer residual toner is collected by the cleaning unit. The toner can also remain on the peripheral surface of the intermediate transfer belt when a patch test is performed. The cleaning unit is configured to collect not only the transfer residual toner but also residual toner generated by the patch test.